Afghan presidential candidate Abdullah Abdullah is leading with 80 percent of votes counted.
According to the Independent Election Commission (IEC), Dr Abdullah has 43.8 percent of ballots cast, followed by Ashraf Ghani with 32.9 percent.
The IEC said the preliminary results suggested it was unlikely there would be an outright winner and that a run-off vote would be held on 28 May.
According to the BBC, the IEC preliminary results were based on nearly 82.6 percent of the vote from the 34 provinces.
The final results are due on 14 May.